The present invention relates to soil stress forces simulating and application devices and methods.
The present invention further relates to devices and methods that both simulate and apply the effects of soil shear stress forces upon test pipe segments and adherent pipeline anti-corrosion protection coatings.
The present invention also relates to devices and methods that both simulate and apply the effects of soil shear stress forces, in combination with the effects of increased internal pipeline operating temperatures upon test pipe segments and adherent pipeline anti-corrosion pipewrap systems.
There exists at the present time an important need for a high soil shear stress-resistant pipeline anti-corrosion system. The widely use pipeline anti-corrosion system usually takes the form of a helically applied tape-like protective outerwrapping. The tape-like component may be applied directly over an unprepared pipeline outer surface, or may, in fact, be overlaid onto a "primer"-coated, pretreated pipeline outer surface.
Other pipeline anti-corrosion protective materials, such as a foam polyurethane material, or the like, may be either sprayed or brushed directly onto a pipeline outer surface prior to inground implantation of the pipe structure.
An important desired characteristic of newly developed pipeline anti-corrosion protective coatings is minimal coating creep in high soil shear stress environments.
In order to attain this coating characteristic, it is deemed extremely important to be able to provide a test apparatus and method that will be capable of both simulating and applying the effects of inground soil shear stress forces upon test pipe segments and adherent pipeline anti-corrosion protective coating systems under pipeline operating temperatures.
Anti-corrosion protective coatings that are to be applied to pipeline structures destined for inground implantation are often subjected to rather severe long-term shearing forces derived from the surrounding soil. The magnitude of these shearing forces depends upon several factors, including amongst others: (a) the type of the soil, (b) the tectonic forces surrounding the implanted pipeline, (c) the size of the pipe, (d) the axial site emplacement and (e) the range of thermal expansion of the pipe as well as its contents under pipeline operating conditions.
In order to understand how each of the above factors affects the overall soil shear stress forces that are imparted to an adherent inground pipeline coating, we first shall consider the forces acting upon implanted pipelines.
Frictional forces acting between the pipeline anti-corrosion protective coating and the surrounding soil are the primary source of soil shear stress. Frictional forces are here defined as the product of the frictional coefficient between the pipeline coating and the soil and the normal force acting around the pipe. As the coefficient of friction depends upon both the nature of the pipeline coating as well as the surrounding soil, it will be found to vary in different applications. Olefin polymer pipeline protective coatings, such as polyethylene, or the like, inherently exhibit lower coefficients of friction, as the proctective tape outer surfaces are smooth and substantially non-adherent.
Other factors having importance in these considerations are the weight of the soil above the pipe, as well as the weight of the pipe, including its contents. In addition, since the normal force will vary depending on the axial position around the pipe diameter, the frictional force, and hence the shearing force, will also be found to vary around the diameter of the pipe.
The result of long-term soil shear forces on a pipeline protective coating is referred to as "soil stress". Soil stress on anti-corrosion protective coatings generally results from the structural shear forces which may also cause the adherent pipeline anti-corrosion protective coating to creep along the pipeline peripheral surface.
Anti-corrosion protective coating creep in these circumstances is, in essence, a long term visco-elastic, or "cold-flow" phenomenon, common to all polymeric substances. The amount of protective coating creep, however, will depend upon the physical properties of the coating. Since the physical properties (i.e. modulus) of the coating, will be temperature dependent, temperature becomes a decisive element in determining the amount of coating creep. At low temperatures, the propensity of the pipeline anti-corrosion protective coating to creep will be substantially reduced, while at elevated temperatures, the likelihood of protective coating creep will be significantly increased, other factors remaining the same.
Previously, it has been necessary to actually field implant large pipeline sections having an anti-corrosion protective coating layer overlaying the outer pipe surface. These implanted test pipe segments are then dug up at desired long time intervals in order to observe the effects of soil stress forces on the anti-corrosion protective coatings.
This prior art ubiquitous method, is clearly a very time-consuming and expensive process, taking from 30 to 90 days, at a minimum to complete, in order to observe the soil stress forces effects on the adherent anti-corrosion protective coatings.